Field of the Invention
The present invention relates to composite joined bodies in which constituent members are joined together through an intermediate member. More particularly, the invention relates to composite joined bodies having a structure which is favorably used for joining together constituent members having different coefficients of thermal expansion, for instance, ceramic members and metallic members.
Related Art Statement
In order to obtain composite joined bodies by butting and joining together members having different coefficients of thermal expansion, there has formerly been known a technique for joining them by direct pressure welding and a technique for joining them through a separate intermediate layer at an interface thereof. For instance, Japanese utility model registration application Laid-open No. 59-160,533 discloses a method of joining a ceramic member and a metallic member through a clad made of a tungsten material or a molybdenum material at an interface thereof as a joining structure.
However, since a residual stress is retained at the joining interface due to difference in thermal expansion in the above-mentioned joining methods, sufficient joining strength cannot be obtained. FIG. 9 shows, by way of example, a metallic member joined to a ceramic member. Assume that the metallic member having a larger coefficient of thermal expansion is joined to the ceramic member having a smaller coefficient of thermal expansion in an intended joined shape before cooling. Once the thus joined composite body is cooled, the metallic member shrinks more than the ceramic member. Thus, as shown in FIG. 9, tensile stress is exerted upon each of the metallic member and the ceramic member at an interface edge "A". As this phenomenon becomes more serious, peeling or cracking at the interface edge "A" will occur.
When a clad made of a tungsten material or a Mo material is used as an intermediate layer, as compared with the above joining of the metallic member and the ceramic member by direct pressure welding, residual stress is reduced to some extent. However, since the intermediate layer is not provided to remove the residual stress at the joining interface, it is not always possible to eliminate peeling and cracking at the interface.
Further, Japanese patent application Laid-open No. 62-182,170 discloses a structure of joining a ceramic member and a metallic member through an insert layer forming a part which is not bonded at least either between a ceramic member and an insert layer or between a metallic member and an insert layer. However, the residual stress is not effectively removed at the joining interface edge because the rigidity of the insert layer is not reduced near the edge portion.